1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, and more particularly to a disk with a perpendicular magnetic recording layer for use in magnetic recording hard disk drives.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. A common type of perpendicular magnetic recording system is one that uses a “dual-layer” media. This type of system is shown in FIG. 1 with a single write pole type of recording head. The dual-layer media includes a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In FIG. 1, the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits.
FIG. 2 is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field Hw acting on the recording layer RL. The disk also includes the hard disk substrate, a seed or onset layer (OL) for growth of the SUL, an intermediate layer (IL) between the SUL and the RL, and a protective overcoat (OC). The IL is a nonmagnetic layer or multilayer structure, also called an “exchange break layer” or EBL, that breaks the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and facilitates epitaxial growth of the RL. While not shown in FIG. 2, a seed layer is typically deposited directly on the SUL to facilitate the growth of the IL. As shown in FIG. 2, the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole (FIG. 1) which is the real write head (RWH) above the disk, and an effective secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the IL and by virtue of its high permeability produces a magnetic mirror image of the RWH during the write process. This effectively brings the RL into the gap of the ARH and allows for a large write field Hw inside the RL.
One type of material for the RL is a granular ferromagnetic cobalt alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity (Hc) media and to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL is achieved by the addition of oxides, including oxides of Si, Ta, Ti, and Nb. These oxides tend to precipitate to the grain boundaries, and together with the elements of the cobalt alloy form nonmagnetic intergranular material. A perpendicular magnetic recording medium with a RL of a CoPtCr granular alloy with added SiO2 is described by H. Uwazumi, et al., “CoPtCr—SiO2 Granular Media for High-Density Perpendicular Recording”, IEEE Transactions on Magnetics, Vol. 39, No. 4, July 2003, pp. 1914-1918. A perpendicular magnetic recording medium with a RL of a CoPt granular alloy with added Ta2O5 is described by T. Chiba et al., “Structure and magnetic properties of Co—Pt—Ta2O5 film for perpendicular magnetic recording media”, Journal of Magnetism and Magnetic Materials, Vol. 287, February 2005, pp. 167-171.
The cobalt alloy RL has substantially out-of-plane or perpendicular magnetic anisotropy as a result of the c-axis of its hcp crystalline structure being induced to grow substantially perpendicular to the plane of the layer during deposition. To induce this growth of the hcp RL, the IL onto which the RL is formed is also an hcp material. Ruthenium (Ru) and certain Ru alloys, such as RuCr, are nonmagnetic hcp materials that are used for the IL.
The enhancement of segregation of the magnetic grains in the RL by the additive oxides is important for achieving high areal density and recording performance. The intergranular material not only effectively decouples intergranular exchange but also exerts control on the size and distribution of the magnetic grains in the RL. Current disk fabrication methods achieve this segregated RL by growing the RL on an IL that exhibits columnar growth of its grains. The columnar growth of the IL is accomplished by sputter depositing it at a relatively high sputtering pressure. However, growth of the RL on this type of IL leads to significant roughness and discontinuities in the RL, and consequently to reduced mechanical integrity of the protective OC. Poor OC coverage, roughness in the RL, and columnar growth of the IL provide a relatively easy path for water and corrosive agents to migrate through these layers and interact with the SUL. Formation of the IL at reduced sputtering pressure can reduce the RL roughness and improve the corrosion resistance of the disk. However, disks with ILs formed at lower sputtering pressure exhibit significantly reduced coercivity and thus poor recording performance.
What is needed is a perpendicular magnetic recording disk that has a granular cobalt alloy RL with additive oxides and that exhibits good corrosion resistance without compromising recording performance.